In the conventional shift-by-wire system that changes/switches a shift position by using a shift position switch mechanism of an automatic transmission, the switch mechanism is driven by a rotary actuator in which a motor is installed. Such a shift-by-wire system has an increased freedom of position arrangement and design of a shifter in a vehicle, because the shifter needs not be mechanically connected with the shift position switch mechanism.
In case that a vehicle is parked on a slope, the gravity dragging the vehicle down on the slope is transmitted via an axle and is applied to an engagement part between a park gear and a park pole in the shift position switch mechanism. Therefore, when releasing the engagement of the park gear and the park pole (i.e., when shifting from a park position to a different position, which may be designated hereafter as a park-release shift), change/switching of a shift position requires a greater torque when the vehicle is parked on the slope than when parked on a flat land. Further, a torque of the motor may change when a power source voltage changes, or when a resistance of motor winding changes according to a motor temperature change. Thus, when the motor is put in a severe condition such as having a low power source voltage, having a high motor temperature or the like, an output torque of the motor may fall down to a smaller value.
In a patent document, Japanese patent No. 5375775 (patent document 1), a technique for securely changing the shift position from the park position (i.e., securely performing a park-release shift) in such a severe condition described above is disclosed, in which a motor controller of the shift-by-wire system first performs a backward rotation operation of the motor in a backward rotation direction relative to a shift position switching direction by a small amount, and then performs a forward rotation operation of the motor in the shift position switching direction, thereby increasing the kinetic energy of a drive object toward the shift position switching direction. The reverse rotation operation of the motor is performed (i) whenever the shift position is changed/switched from the park position, or (ii) when the sever condition of the power source voltage or the motor temperature is detected, or (iii) when the vehicle is parked on the slope.
In the motor controller disclosed in the patent document 1, every time the shift position is changed/switched from the park position, the backward rotation of the motor is performed, which takes time and leads to an undesirable increase of a shift position switch time. Further, when the power source voltage is relatively low, or when the motor temperature is relatively high, or when the vehicle is parked on the slope, the backward rotation of the motor is always performed, i.e., even in case that the shift position change/switching is successfully performable without the backward rotation of the motor.